1. Field of the Invention
The invention relates to the purification of a gas stream in a pressure swing adsorption system. More particularly, it relates to a process for enhancing the recovery of purified hydrogen in such a system.
2. Description of the Prior Art
The pressure swing adsorption, i.e., PSA, process is an advantageous means for separating and purifying hydrogen contained in gas streams containing carbon dioxide and other impurities. The PSA process is well known in the art, as evidenced for example by the Wagner U.S. Pat. No. 3,430,418 and the Fuderer U.S. Pat. No. 3,986,849. Gas streams containing relatively high concentrations of carbon dioxide, e.g., between about 15% and about 40% by volume, can be successfully treated by the PSA process. The hydrogen in the feed gas is separated and purified not only from the carbon dioxide, but from other components that may be present in the feed gas, such as methane, ethane, carbon monoxide, nitrogen and the like.
It is also within the skill of the art to process the feed gas stream containing such a relatively high concentration of carbon dioxide to a pretreatment absorber column wherein most of the carbon dioxide is absorbed in a suitable liquid. The thus--pretreated gas stream is subsequently further purified in a PSA system. Such combined absorption and PSA purification has the advantage that one part of the carbon dioxide content of the gas stream can be recovered as a relatively pure, e.g., 98% plus, CO.sub.2, often a desirable product, in addition to the hydrogen being purified. A further advantage of the combined purification approach is that the size and cost of the PSA system are reduced thereby. In an illustrative example based on the use of an eight bed PSA system, the size of the PSA bed for treating a feed gas containing 22.9% CO.sub.2, 70% hydrogen and 7.1% other components was 20 m.sup.3, whereas the bed size was reduced to 12.2 m.sup.3 when the feed gas was pretreated to reduce the CO.sub.2 content to 3.0%, with the hydrogen content being 88% and other components 9.0%. Hydrogen recovery was increased from 86.3% for the untreated feed gas to 88.0% for the pretreated gas stream based on the hydrogen content of the feed gas. The feed flow rate was reduced from 20,000 Nm.sup.3 /h for the untreated feed gas to 15,900 Nm.sup.3 /h for the pretreated gas stream. This reduced flow rate, together with the reduced CO.sub.2 content of the gas passed to the PSA unit, contributed to the indicated reduction in bed size when the feed gas is pretreated in an absorption zone to remove a substantial portion of the carbon dioxide content thereof.
The combined absorption PSA purification technique has, of course, the drawback of additional investment and operating costs associated with the liquid absorption system. There is, therefore, a desire in the art to obtain the higher hydrogen recovery and the reduced absorbent bed size of the combined purification approach, but at a reduction in the incremental costs of the liquid absorption system. Additional improvements in hydrogen recovery are likewise desired so as to further enhance the advantages associated with the PSA-hydrogen process in its important commercial applications using feed streams having relatively high concentrations of carbon dioxide.
It is an object of the invention, therefore, to provide an improved PSA process for the separation and purification of hydrogen from gas streams having relatively high concentrations of carbon dioxide.
It is another object of the invention to enhance product recovery in PSA-hydrogen process operations.
It is a further object of the invention to provide hydrogen recovery improvement and absorbent bed size reduction, as achieved by the combination absorption--PSA system, at lower operating and investment costs for the incremental, pretreatment liquid absorption system.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.